1. Field of the Invention
This invention relates to a soil amendment, and more particularly, to a soil amendment made from waste products that is useful for reclaiming mine spoils and for enriching agricultural, horticultural, and turf grass soils without releasing undesirable levels of heavy metals into the environment.
2. Description of the Prior Art
The combustion of coal at conventional power plants results in a variety of residues that require disposal, including bottom ash, fly ash, and flue-gas desulfurization waste. Bottom ash is the residue from coal combustion that remains in the boiler. It consists of both fine- and coarse-grained materials and is generally a combination of ash and slag. Fly ash is the residue from coal combustion that passes out from the boiler with the flue-gas stream. It is composed predominantly of fine particles having a high clay content and some free carbon. Fly ash particles are either collected in emission-control devices, such as electrostatic precipitators, baghouses, or mechanical collectors, or are released from the stack. Flue gas desulfurization (FGD) wastes contain sulfur oxides captured from the flue gas, as well as calcium or sodium sorbent materials that react with the sulfur oxides. FGD wastes are collected from a wet scrubber or from particulate control units located downstream from the boiler.
Raw bottom ash, fly ash and FGD wastes are commonly disposed of in landfills or ponds in the U.S. When these materials are deposited in large amounts on vegetated surfaces, the vegetation there tends to die, and revegetation is difficult.
Disposing of coal combustion residues, particularly FGD waste, is increasingly a problem. Millions of tons of FGD waste are landfilled or ponded each year. The nature of this waste varies with the desulfurization process used. The most common FGD waste is scrubber sludge. Scrubber sludge is produced from slurries of water and lime or limestone that are mixed with the flue gas stream in large vessels called wet scrubbers. The resulting waste sludge is a semi-solid material composed of ash, water, and compounds of calcium, magnesium, and sulfur. With added processing, it may be dried or oxidized to a more disposable form. Another common FGD waste is spray dryer waste. Spray dryer waste is produced from water and lime slurries that are mixed with the flue gas stream in large vessels called spray dryers. The discharge in this case is a dry powder product having a composition similar to that of a wet scrubber sludge, but without the high water content. Still another FGD waste is duct or furnace injection waste. Injection waste is produced when dry lime compounds are injected directly into the furnace or into flue-gas ductwork or downstream equipment, and then removed from the flue gas stream before the gas enters the stack. Conventional furnace and duct injection waste is dry, very fine, and tends to contain large amounts of unreacted lime. Waste from more advanced duct injection processes may be coarser in size, particularly if the process employed is that of Holter et al (U.S. Pat. No. 4,061,476) or that of the Inventor (U.S. Pat. No. 4,786,484).
Alkali oxides with vermiculite or perlite are employed in several FGD processes. These include processes by Wilson (U.S. Pat. No. 3,882,221), by Holter et al (U.S. Pat. Nos. 4,061,476 and 4,201,751), and by the Inventor (U.S. Pat. Nos. 4,721,582 and 4,786,484). Wilson describes the use of sodium or potassium alkali. Holter et al teaches the use of several materials as sorption agents, with the preferred embodiment being iron oxide, and describes a typical duct-injection process. The Inventor teaches the use of divalent metal oxides or hydroxides, with the preferred oxide being magnesium oxide that can be injected into a gas stream. In all processes, no recognition is given to the possibility of using the waste FGD materials as useful products, and no mention is made of soil amendments. In fact, concerns about disposing of the waste FGD materials led each inventor to propose the thermal or chemical decomposition or regeneration of the waste materials to make the wastes more environmentally acceptable. It should be noted that the use of FGD wastes from processes based on such oxides as sodium, potassium, and magnesium, moreover, are probably not appropriate for soil amendments, owing to their high solubilities in water.
When coal is mined, large amounts of mining wastes, commonly called spoils, are produced. These spoils are objectionable because they are unsightly, they support little or no vegetation, and they often lead to run off of highly acidic wastewater streams. Spoils are a particular problem at surface coal mines where many square miles of once pristine forests and meadowland become lifeless wastes. In many locations, surface strip mines that were abandoned 40 to 60 years ago remain barren. A need exists to revegetate these areas and to bring the ecosystem back into balance.
Fly ash has been examined in the past as an amendment to acidic surface mine spoils and mine wastes. Fly ash has been shown to be effective in increasing spoil pH, allowing the establishment of permanent vegetative cover on previously barren sites. However, because of the high acidity of the spoils, very high ash addition rates have been usually required and these, in turn, have often caused an increase in the soluble salt concentrations in the spoils. The principal concerns in the use of fly ash have been groundwater contamination from leachates and the distribution of trace elements found in the ash.
The environmental impacts associated with the disposal of scrubber sludge are similar in many ways to those associated with fly ash. In fact, fly ash is often a component of scrubber sludge, and the two wastes are commonly placed in the same disposal basin. Scrubber sludges are characterized by high pH, low levels of nitrogen and phosphorus, high soluble salt contents, and higher than normal concentrations of trace elements. As with fly ash, concerns with scrubber sludge center on possible groundwater contamination and elevated concentrations of trace elements, such as boron or selenium. An added concern with many scrubber sludges is the presence of high concentrations of sulfite. With anoxic conditions, H.sub.2 S gas can be produced. With aerobic conditions, high levels of sulfite can significantly increase the oxygen demand, owing to the oxidation of sulfite to sulfate, in surface and ground waters affected by these wastes.
Past efforts have centered on developing improved procedures for disposing of coal combustion residues in an environmentally acceptable manner. The nature of the residues, for example, the dusty nature of fly ash and the strongly alkaline nature of lime wastes, has often made simple land use undesirable. Approaches to make land-fill and road-base usage acceptable have been proposed by Webster in U.S. Pat. Nos. 4,028,130 and 4,354,876, by Burns and Gremminger in U.S. Pat. No. 5,277,826, and by others. In U.S. Pat. No. 4,028,130, Webster teaches the addition of digested sewage sludge to coal combustion residues to produce a curable material suitable for land-fill usage. Burns and Gremminger propose a similar process involving sewage sludge. In U.S. Pat. No. 4,354,876, Webster describes a process for handling dry scrubber wastes. In this process, water is mixed with dry fly ash-containing scrubber wastes and the water-waste combination is compacted at the land-fill site.
Many new soil amendment products have been proposed. However, only a few involve the usage of coal combustion residues as a component of a soil amendment. Most noteworthy of these is a process described by Rehbein and Montain in U.S. Pat. No. 5,468,277, wherein a powder product comprised of fly ash and desulfurization residue from a spray dryer is added to agricultural soils. Rehbein and Montain make no special additions to their powdery soil amendment. The soil amendment of the present invention differs from Rehbein and Montain's invention in several important aspects. First, the soil amendment of my invention contains individual expanded vermiculite and/or perlite particles. The expanded vermiculite or perlite particles are important because they provide a structural support and retention means for the other components in the amendment and for moisture, they aerate the soils, and they possess electrolyte properties important to the soil environment. Second, my amendments are not powders, but are, instead, individual particles or are pellets prepared from these individual particles. Third, my amendments do not require any special application procedures. Commercial application equipment can be employed. Fourth, unlike Rehbein and Montain's invention, in which the amendments contain water at a weight ratio of 10:8 to 20 to form a slurry, my amendments do not contain added water and are employed essentially dry.
Another example of a soil amendment made from coal combustion residues is taught by Higa (U.S. Pat. No. 4,985,060). Higa describes a beneficial soil amendment consisting of coal ash and slag, to which 5 to 20 wt % zeolite is added, as well as optional microorganisms and a binder component. The Higa soil amendment, however, does not include FGD waste. Also, by not including FGD materials, the ability of the amendment to neutralize acidic soils is limited and larger amounts must be used for equivalent liming levels. Unlike the current invention, where the vermiculite or perlite are wastes from the FGD process, the zeolites in the Higa amendments must be purchased specifically for that purpose.